NEW EVIDENCE OF A LARGE THEROPOD DINOSAUR FROM THE YELLOW CAT MEMBER (BARREMIAN, EARLY CRETACEOUS) OF THE CEDAR MOUNTAIN FORMATION, UTAH

A number of theropod taxa have been recovered from the Barremian aged Yellow Cat Member of the Cedar Mountain, e.g., Nedcolbertia, Falcarius, and Utahraptor. All, however, are relatively small, with Utahraptor being the largest at 4.2 meters long. Isolated large teeth and fragmentary bones have hinted at the presence of larger theropods in this member. Here, we provide substantial new evidence of large theropods in this unit based on specimens from three sites in eastern Utah.

The Grayash site has yielded 16 large, robust teeth, with crowns up to 52 mm long, of a large theropod. The nearby MayRae Quarry, approx. 300m away, has produced a 95 mm long proximal phalanx of manual digit as well as eight closely associated theropod dorsal ribs, up to 107 mm long. The ribs are gracile with a large pneumatic fossa occupying the posterior surface of the rib heads. The proximal rib shafts are pneumatized via a large foramen at the lateroventral border of that fossa. The bone in this area is as thin as 0.5 mm.

An isolated, posterior dorsal vertebra, BYU 53409, from BYU Locality 1533, is 330 mm tall with a 120 mm wide centrum, pertains to a relatively large theropod. Unfortunately, the vertebra is poorly preserved due to severe preburial weathering and insect damage, precluding a more refined taxonomic identification.

Taken together, these theropod elements corroborate the presence of at least one large theropod taxon with an estimated length of 6 m. Considering the abundance of medium-sized sauropods and a range of good-sized ornithopods in the Yellow Cat Member fauna, a theropod the size of an average Allosaurus is not unexpected. The question remains as to why, after decades of intensive collecting, the remains of large theropods are so scarce in the basal Cedar Mountain Formation.